Communication cables employing optical fibers are widely used in the telecommunications industry. In particular, multifiber cables are widely used for long distance telephone communications, interexchange telephone applications, and other telephony and data transmission applications. Fiber optic cables are also being incorporated into cable television networks in place of more traditional coaxial cables. Optical fibers may permit long distances between signal repeaters or eliminate the need for such repeaters altogether. In addition, optical fibers offer extremely wide bandwidths and low noise operation.
A fiber optic cable typically includes a core and an outer protective jacket. A plurality of optical fibers are contained within the core. For a typical cable, such as used for long distance communications, the fibers are maintained in a loose-buffered relationship within the core to thereby isolate the fibers from strain imparted to the cable as the cable is installed and thereafter. A typical loose-buffer cable, such as available from Siecor of Hickory, N.C. under the designation MINIBUNDLE.TM., includes a series of plastic buffer tubes stranded around a central support member in a concentric layer. The buffer tubes are identifiable based upon a unique color imparted to the plastic material. The individual optical fibers also include a color-coded coating thereon to enable identification of a particular individual fiber from the other fibers within a buffer tube of the cable.
A loose-buffered cable offered by AT&T under the designation LIGHTPACK LXE.RTM. is an effort to achieve a higher fiber packing density than available with a typical stranded multiple buffer tube cable. The AT&T cable includes a relatively large single central buffer tube which contains the plurality of optical fibers. U.S. Pat. No. 4,844,575 to Kinard et al. discloses such a cable wherein two groupings of individual fibers are formed within the single central buffer tube by a color-coded wrapping yarn surrounding predetermined ones of the individual optical fibers. Thus, the single central buffer tube cable includes color-coded yarns and color-coded optical fibers thereby providing two levels of color coding to assist a technician in identifying a particular optical fiber within the cable. This two-level color coding scheme is similar to that used in the stranded multiple buffer tube cable described above.
Color coding is typically used to identify one optical fiber from a plurality of such optical fibers in a multifiber cable. Since the number of distinctive basic colors are limited to typically twelve, color-coded optical fibers are typically segregated into groupings that are themselves identified by the same basic twelve colors. For example, in a 96-fiber cable of the stranded multiple buffer tube type, eight color-coded buffer tubes may each contain twelve individually color-coded optical fibers. Similarly, for a 96-fiber optic cable having a single central buffer tube, presumably groups of twelve color-coded fibers may be arranged into eight bound groupings by eight respective individually color-coded yarns.
Unfortunately, an emerging cable television architecture requires a relatively small number of grouped fibers, typically three or four, to be identified, accessed and connected at a drop point. In addition, a large number of spaced apart drop points are typically required along the cable route and, hence, a large number of individual fibers are also required in the cable. Conventional stranded multiple buffer tube cables and single central buffer tube cables are unacceptable for such an architecture. For example, if groups of four fibers are desired in a 96-fiber cable, an exceedingly large number of buffer tubes, i.e., twenty-four, would be needed in the multiple buffer tube cable, greatly reducing the packing density of the cable and concurrently increasing the cost of the cable. Higher fiber count cables would require an even greater number of buffer tubes. For the central buffer tube cable, current designs are limited to only about eight groups; therefore, the maximum number of fibers in such a cable would be limited to only thirty-two.
Another important aspect of a fiber optic cable for the emerging cable television architecture, in addition to being able to readily identify an individual fiber, is the ability to access the relatively small grouping of optical fibers without damaging adjacent fibers. In other words, it is important that disturbance to adjacent fibers be minimized to prevent the possibility of damage to the fibers. In this respect, the multiple buffer tube cable offers an advantage over the central buffer tube cable. A single buffer tube may be accessed in the multiple buffer tube cable while the remainder of fibers within other buffer tubes are undisturbed. In contrast, entry into the single central buffer tube is likely to increase the risk of damaging adjacent fibers since all of the fibers are contained within the single buffer tube.